drm/amdgpu: During compute disable GFXOFF for Sienna_Cichlid

Workaround to fix the soft hang observed in certain compute
applications.

Acked-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Harish Kasiviswanathan <Harish.Kasiviswanathan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>